The present disclosure relates to a coupling arrangement or coupling apparatus between a light emitting diode (LED) and a light guide, and more particularly, to improving the coupling efficiency between the components. The coupling apparatus in this particular application is a compact fluorescent lamp (CFL), for example, where there is a desire to intermix red color light with white light emitted from the CFL, although the disclosure may find more general application in related environments where a high efficiency or coupling is desired.
Color rendering of saturated red color of a standard CFL can be enhanced by a combination with red light from an LED. For example, commonly owned U.S. Pat. No. 7,543,957 is directed to such an arrangement, the disclosure of which is expressly incorporated herein by reference. High efficiency red LED's are temperature sensitive so there is a need to thermally insulate the LED from the CFL. The noted '957 patent mounts the LED in a base portion of the lamp assembly in order to segregate the thermally sensitive LED from the elevated temperature associated with the CFL. A light guide is employed to convey the red light from the LED in the base portion of the lamp assembly to a region adjacent the discharge tube(s) of the CFL where effective mixing with the white light emitted by the discharge tube(s) is attained.
The higher the in-coupling, transferring, and out-coupling efficiencies of a light guide, the higher the resultant effect on saturated red color rendering of the lamp. Improved efficiencies associated with (i) light entry into the light guide, (ii) along the length of the light guide, and (iii) upon exiting of the light guide, result in more red color intermixing with the white light from the CFL from a particular LED driven at a particular wattage.
In-coupling efficiency is deemed to be the most critical junction in the light path. Geometric incompatibility of the radiation mode of the source (here, the LED) and the guided mode (here, the light guide) indicates that substantial areas for improvement are required. By radiation mode is meant the angular and spatial distribution of light emitted from the LED. Where large light guides are used in conjunction with the light guide (that is where the diameter of a light guide is larger than the size of the LED source and its primary optics), the problem simplifies to matching LED radiation characteristics, usually lambertian, along with the limited acceptance angle or (NA of the light guide). Coupling efficiency is measured as a percentage of the light exiting the light guide relative to the intensity of the light emitted by the LED. Known coupling arrangements range exhibit an efficiency that ranges between 35% to approximately 50%.
Various coupling arrangements are suggested in the prior art. For example, one prior art arrangement provides butt coupling or contact between a polished end of the light guide and an integrated optical arrangement associated with the LED. Alternatively, a cavity or recess is formed around the LED to collect the light into the light guide such as shown and described in U.S. Pat. No. 7,279,345. Still another arrangement uses lens coupling where a special geometry lens, lens holder, etc. are interposed between the LED and the light guide (See, for example, U.S. Pat. No. 5,732,176). Higher coupling efficiencies are attained when compared to the butt coupling arrangement. Still another coupling arrangement is a reflector-type coupling and heretofore the designs are based on classic reflector designs used with incandescent light sources which are not easily incorporated into the LED-light guide environment. Typically, the reflector includes a metallic reflective layer and also uses air as a propagation medium within the reflector Eliminating an air gap between the light source and the light guide is desirable because such a structure would reduce the Fresnel reflections on the LED-air and air-light guide interfaces, and also enhances the application of total internal reflection (TIR) on the reflecting surface.
Consequently, a need exists for improved coupling efficiency, and also consideration of commercialization of such an arrangement that limits the cost to manufacture while providing a repeatable solution for both ease of assembly and consistent results.